Method and apparatus for line testing

ABSTRACT

A method of testing a communication line includes applying a voltage as a function of time on the communication line and measuring at least a first current and a second current flowing via the communication line. The second current is measured at a different point in time than the first current. The method includes deciding whether a given terminal element is connected to the communication line based on the first current and the second current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned U.S. patent applicationSer. No. 11/489,131, filed on Jul. 18, 2006, entitled “METHOD ANDAPPARATUS FOR LINE TESTING,”, and U.S. patent application Ser. No.11/489,376, filed on Jul. 18, 2006, entitled “METHOD AND APPARATUS FORLINE TESTING,” which are both herein incorporated by reference.

BACKGROUND

In general, line testing is employed in wired communication networks forline testing of the respective communication lines. In suchcommunication networks, terminal devices located in the premises of acustomer (also referred to as subscriber) are connected with a centraloffice via such communication lines. An example for a correspondingcommunication network is a public switched telephone network (PSTN)wherein terminal devices like telephones or facsimile devices areconnected with the central office via a pair of copper lines. The copperlines are commonly referred to as tip line and ring line. In the recentyears, data transfer services, such as integrated services digitalnetwork (ISDN) or digital subscriber line (DSL) (e.g., asymmetricdigital subscriber line (ADSL)) have been employed over these copperlines, for example to provide access to the internet.

In such systems, in the central office the copper lines are usuallyconnected with a subscriber line interface circuit (SLIC) on acorresponding line card which, depending on the services, is referred toas a PSTN line card, DSL line card, or the like.

SUMMARY

One embodiment provides a method of testing a communication line. Themethod includes applying a voltage as a function of time on thecommunication line. The method includes measuring at least a firstcurrent and a second current flowing via the communication line. Thesecond current is measured at a different point in time than the firstcurrent. The method includes deciding whether a given terminal elementis connected to the communication line based on the first current andthe second current.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates an example embodiment of an equivalent circuit of acommunication line.

FIG. 2 illustrates an example embodiment of a communication system.

FIGS. 3A and 3B illustrate equivalent circuits of possible off-hooktermination.

FIG. 4 illustrates a block diagram of one embodiment of one apparatus.

FIG. 5 illustrates a flow diagram of one embodiment of a method.

FIG. 6 illustrates voltage and current curves occurring during anexemplary execution of the method embodiment of FIG. 5.

FIG. 7 illustrates a graph illustrating the measurement of a signatureresistance according to one embodiment.

FIG. 8 illustrates an oscilloscope measurement during an exemplaryexecution of the method embodiment of FIG. 5.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

Embodiments relate to methods and apparatuses for line testing ofcommunication lines.

In the following, methods and apparatuses for line testing according toembodiments will be described. In order to provide a clearerunderstanding of the embodiments, first an exemplary environment wherethe embodiments may be used will be described with reference to FIGS.1-3.

In FIG. 1 illustrates an example equivalent circuit embodiment for acopper line pair comprising a tip line A and a ring line B between aline card located in a central office and a subscriber. The line cardcomprises a SLIC 1 and resistors R1-R7 and capacitances C1, C2 forconnecting SLIC 1 to ring line A and tip line B. As explained furtherbelow in detail, line cards may comprise a plurality of subscriber lineinterface circuits and also other elements, such as coder/decoder(CODECs).

In the example circuit embodiment illustrated in FIG. 1, tip line A isconnected to SLIC 1 via resistances R3 and R1, whereas ring line B isconnected to SLIC 1 via resistances R4 and R2. Example suitable valuesare 30Ω for resistances R1 and R2 and 20Ω for resistances R3 and R4,although these values may be different in different embodiments.Furthermore, capacitances C1 and C2 are connected between tip line andring line, respectively, on the one hand and ground on the other hand.An example suitable value for capacitances C1 and C2 is 15 nF.

Resistances R1 through R4 stabilize and protect SLIC 1 and together withcapacitances C1 and C2 form filters for filtering out unwanted frequencycomponents.

Furthermore, tip line A is coupled with a common mode voltage VCM viaresistances R5 and R6, and ring line B is connected with common modevoltage VCM via resistances R8 and R7. Example suitable values are 10 MΩfor resistances R6 and R8 and 47 kΩ for resistances R5 and R7. Asresistances R6 and R8 have large values, only negligible current flowsbetween tip line A and VCM and ring line B and VCM. However, as will beexplained later in more detail, R5 and R6 as well as R8 and R7 may serveas voltage dividers which enable a measurement of large voltages on tipline A and ring line B.

A section designated “line and leakage” in FIG. 1 includes theequivalent circuit of the copper line itself. Equivalent circuit in thiscase means that the intrinsic capacitances and resistances of the copperline A, B are depicted as separate capacitances and resistances.

In particular, two resistances each designated Rline/2 are illustratedin the “line and leakage” section of FIG. 1 representing the resistanceof tip line A and ring line B, such that the overall resistance of thecommunication line is Rline.

Leak resistances and capacitances are also illustrated in the “line andleakage” section of FIG. 1, namely Rtg as a leak resistance between tipline A and ground, Ctg as a leak capacitance between tip line A andgroung, Rrg as a leak resistance between ring B and ground, Crg as aleak capacitance between ring line B and ground, Rtr as a leakresistance between tip line A and ring line B, and Ctr as a leakcapacitance between tip line A and ring line B. In a fault free state,the resistances Rtg, Rrg, and Rtr are very large (e.g., several tens ofMΩ), and the capacitances Ctg, Crg, and Ctr are comparatively small(e.g., in the order of 47 nF per km of the line). Therefore, nosignificant direct current (DC) and only small alternating current (AC)currents may flow via these connections.

In addition to what is illustrated in FIG. 1, tip line A and/or ringline B may be accidentally connected to what is called a foreignvoltage, for example if tip line A is connected with a copper wire of adifferent communication line such that the voltage applied to thisdifferent communication line is also coupled with tip line A. However,these foreign voltages are not necessary for the understanding of thedescribed embodiments and will therefore be not discussed further here,but is discussed in detail in the above incorporated patent applicationSer. No. 11/489,131. However, it should be noted that the describedembodiments may be combined with methods or devices for evaluating,detecting and handling such connections to foreign voltages, for exampleby performing the necessary measurements one after the other.

As illustrated in FIG. 1, on a subscriber side (i.e., at a far end oftip line A and ring line B in customer's premises one or more terminaldevices like telephones, DSL modems, facsimile devices, or the like maybe present. Such devices have typical signatures or termination, threeof which are depicted in FIG. 1. The termination of the line provided bya telephone in an on-hook state (i.e., a state where the telephone isinactive) is indicated at 5. In this case, the telephone may berepresented by a resistance Rr and a capacitance Cr connected in seriesbetween tip line A and ring line B. In contrast, when the telephone isin an off-hook state, for example when a telephone call is made, thetelephone may be represented by the equivalent circuit indicated at 6,with a pair of Zener diodes Do1 and a resistance Ro1 connected inseries. Finally, if a DSL modem is present, a corresponding signaturemay be provided such that the presence of the DSL modem may be detected.An example for such a signature is indicated at 7 and comprises a pairof Zener diodes Dd, a resistor Rd, and a capacitance Cd connected inseries. According to one embodiment, the DSL signature 7 may compriseZener diodes with an example suitable breakthrough voltage of 6.8 V, aresistor Rd with an example suitable resistance of 33 kΩ, and acapacitance Cd having an example suitable value of 470 nF.

In the context of this application, the term “termination” designatesthe equivalent circuit of a device connected to a communication line,whereas “signature” designates elements provided to be connected to thecommunication line for identification purposes. A signature may form atermination or a part thereof, and electrically they are the same (i.e.,they represent circuit elements connected to the communication line).

A Zener diode is a kind of breakthrough diode that permits current toflow in the forward direction like a normal diode, but also in thereverse direction if the voltage is larger than the specified breakdownvoltage or Zener voltage. In contrast thereto, a conventional solidstate diode will not permit current flow if reversed biased below itsreverse breakdown voltage. If the breakdown voltage is exceeded, aconventional diode will be destroyed in the breakdown due to excesscurrents which cause overheating. A Zener diode is designed to have areduced breakdown voltage, wherein the diode will not be destroyed whenthe breakdown voltage is reached. In other words, a reverse biased Zenerdiode will exhibit a controlled breakdown wherein the current flows toan extent to keep the voltage across the Zener diode at the Zenervoltage (e.g., 6.8 V in case of a suitable example DSL signature 7depicted in FIG. 1).

In the central office illustrated on the right side in FIG. 1, accordingto embodiments a “line testing” procedure is regularly performed toobtain data regarding the copper lines connected to the central office.In particular, in such line testing procedure the values of thecapacitances and resistances in the “line and leakage” section of FIG. 1may be measured, the values in turn then being used for detectingpossible faults in the line, and the signatures and terminations in thecustomer's premises, for example those illustrated in the “subscriber”section of FIG. 1 may be detected so that, for example, the presence orabsence of a DSL modem may be determined.

The embodiments which will be presented in the following relateprimarily to the latter aspect (i.e., to determining a signature ortermination), for example determining whether a DSL signature likesignature 7 in FIG. 1 is present. However, when determining thesignature, also information regarding the leakage capacitances andresistances may be obtained, and furthermore the embodiments presentedmay easily be combined with measurements for determining the leakresistances and capacitances illustrated to a greater extent.

To illustrate embodiments more precisely, in FIG. 2 an exampleenvironment in which such embodiments are useful is depicted.

In particular, in a central office 8 illustrated in FIG. 2 a pluralityof line cards 9, 10, 11 are provided, wherein line cards 9 and 11 areconnected to respective telephones 17 via copper lines A, B like thecopper lines A, B illustrated in FIG. 1. Telephones 17 may be located incustomer's premises and, in an on-hook state, terminate lines A, B asindicated by 5 in FIG. 1.

Line card 10 is also connected via copper lines A, B with customer'spremises. However, in this case both a telephone 15 and an xDSL modem,for example an ADSL modem 16, are connected to the copper lines A, B viaa splitter 12. Such splitters have the function to provide a connectedtelephone like telephone 15 with the corresponding telephone signals anda DSL modem like xDSL modem 16 with the corresponding DSL signals. Toachieve this, for example a low pass filter 13 and a high pass filter 14are provided. These filters may be used since usually DSL signals aretransmitted in frequency bands having higher frequencies than frequencybands for transmission of telephone signals.

Furthermore, in splitter 12 a DSL signature 7 corresponding to the DSLsignature 7 of FIG. 1 is provided. With the help of this signature,embodiments may determine that a DSL modem is connected to line card 10.Therefore, when line testing is performed in central office 8,embodiments may determine which line cards are connected with telephonesonly and which line cards are connected to telephones and DSL modems. Afurther possibility is a line card only connected to a DSL modem, whichis not illustrated in FIG. 2.

The embodiments of the present invention are not only applicable tocentral office equipment, but corresponding line cards or connectionsmay also be located, for example, in a private branch exchange (PDX)which is a central unit for a telephone system, for example, of acompany or a firm. Also, as already indicated, several terminal deviceslike telephones or DSL modems may be connected to a single line card,and of course more or less line cards than illustrated in FIG. 2 may beprovided.

Different signatures from those illustrated in FIG. 1 exist. Forexample, FIGS. 3A and 3B illustrate two further possible off-hookterminations for telephones which may be present instead of thetermination 6 of FIG. 1. The terminations illustrated in FIGS. 3A and 3Bboth comprise a resistor Ro2 or Ro3 and a diode Do2 or Do3 and differ inthe polarity of the diode Do2, Do3 with respect to tip line A and ringline B.

In general, the embodiments discussed in the following apply voltages totip line A and ring line B and measure currents flowing via tip line Aand ring line B. Voltages may be applied as plateaus (i.e., constantvoltages) or as ramps (i.e., varying voltages) or a combination of thetwo.

For detecting signatures like DSL signature 7 of FIG. 1, embodimentstake the non-linear nature of the signature into account. For example,for the signature 7 of FIG. 1 or any other signature comprising a Zenerdiode or a similar device, measurements may be performed at a firstvoltage where a Zener diode or a similar element in a signature is in anon-conducting state and therefore decouples the signature from tip lineA and ring line B, and a second voltage where the Zener diodes or otherelement is in a conducting state and therefore the signature is coupledto tip line A and ring line B, and currents flowing via thecommunication line may be measured at both first voltage and secondvoltage. Based on the currents measured, the presence of a signature maybe determined.

The first voltage and second voltage may in particular be part of one ormore voltage ramps from a respective starting voltage to a respectiveend voltage. A plurality of such voltage ramps having different slopesand/or different signs may be used within embodiments, for example tocancel out offsets.

In other embodiments, an element in a termination like the diodes Do2and Do3 in FIGS. 3A and 3B may conduct current only in one direction. Todetermine such a termination, embodiments apply a first voltage having afirst polarity to a corresponding communication line, apply a secondvoltage having a second polarity different from the first polarity tothe communication lines, and measure the current flowing via thecommunication lines in both cases. Again, by comparing the currentsmeasured, the presence of a corresponding termination or signature maybe detected.

While in some embodiments the principle as described above may becarried out with dedicated test equipment comprising voltage sources andcurrent meters for applying voltages to tip line A and ring line B andmeasuring corresponding currents, in other embodiments line cards whichin normal operation are used for handling the communication via tip lineA and ring line B are used for carrying out the measurement. In thiscase, no dedicated test equipment is needed, the embodiments may beimplemented in a cost effective manner and therefore tests may beperformed more frequently.

A corresponding embodiment is illustrated in FIG. 4, wherein a line cardaccording to this embodiment is depicted.

In addition to SLIC 1 connected with tip line A and ring line B alreadydescribed with reference to FIG. 1, a coder/decoder (CODEC) 2 isprovided on the line card. The embodiment of FIG. 4 makes use of thefact that SLICs like SLIC 1 in FIG. 4 nowadays often have the capabilityof generating voltages and currents and measuring currents, whereas manyCODECs have the capability of measuring voltages, for example viadedicated pins which in case of FIG. 4 are designated IO1 and IO2.

CODEC 2 in the embodiment illustrated in FIG. 4 comprises a digitalsignal processor 3 having digital-to-analog and analog-to-digitalconversion capabilities. Such a CODEC with a digital signal processormay be used to convert the measurement data provided by SLIC 1 todigital data for further processing and also to generate analog AC or DCvoltages or current signals which are then output to tip line A and/orring line B via corresponding line drivers in SLIC 1.

Furthermore, as indicated in FIG. 4, SLIC 1 according to the embodimentillustrated has the capability of measuring transversal currents (i.e.,currents flowing via tip line A and ring line B). SLIC 1 may furtherhave the capability of also measuring longitudinal currents, which aremeasured only on tip line A or only on ring line B. However, for manyembodiments, this capability is not necessary.

As also illustrated in FIG. 4, resistors R5 and R6 on the one hand andR7 and R8 on the other hand serve as voltage dividers and arecorrespondingly connected with pin IO1 and IO2 of CODEC 2. These voltagedividers enable also large voltages on tip line A or ring line B to bemeasured without overloading CODEC 2.

As already explained before, the line card may comprise more than onesuch SLIC/CODEC combination, in particular a plurality of thesecombinations so that a plurality of pairs of tip line and ring line maybe connected to the respective line card. Furthermore, a plurality ofline cards may be installed in a host like also illustrated in FIG. 2.The host is provided with data processing capabilities for controllingthe line cards. For communicating with such a host, an interface 4 isprovided in CODEC 2. For carrying out the measurements necessary in theembodiments, according to an embodiment, a corresponding software isdownloaded to a memory of the host or on the line card directly suchthat when the software is run, the line card of the embodiment of FIG. 4is controlled such that the corresponding measurements and calculationsare performed.

The embodiment of FIG. 4 and similar embodiments are easy to realizebecause line cards already present are used for carrying outembodiments, such that no additional hardware is needed. In embodiments,it is sufficient to download a corresponding measurement software into afirmware memory of the host or of the line card in order to be able tocontrol the line card accordingly.

Next, a more detailed embodiment of a method will be described withreference to FIGS. 5 and 6, wherein FIG. 5 illustrates a flow diagram ofthe method embodiment and FIG. 6 illustrates the corresponding voltagesand currents for the method embodiment. At 27, the line to be tested ischecked to determine if the line is in an idle state. An idle state inthis case designates a state in which a telephone or a similar terminaldevice on far end of the line is not active (i.e., in the on-hook statementioned above). In the on-hook state a telephone has a large capacityCr (see FIG. 1), and consequently no DC current flows even if a voltageis applied to the line, for example a voltage of 48 V which is astandard voltage between tip line A and ring line B for PSTN systems andwhich voltage usually provides a power supply for the terminal device.On the other hand, if the line is not idle (i.e., in an off-hook state)a significant DC current may be detected when such a voltage is appliedsince in this case, for example in the termination 6 in FIG. 1, thediodes Do1 are conducting. Therefore, by applying such a voltage, it maybe easily detected if the line is in an on-hook state or in an off-hookstate. If the line is not idle, at 28 a wait is performed for a certaintime, for example half an hour, and then step 27 is repeated. Thisensures that the line testing which follows is not performed while aterminal device like a telephone is used since the test procedure couldinterrupt or disturb, for example a conversation between the user of thetelephone device. Steps 27 and 28 may also be omitted if it desired toperform the test in any case respective of whether a telephone is usedor not.

If the line is found to be idle at 27, at 29 an initialization to afirst constant phase is performed. In other words, the voltage on tipline A and ring line B (in case of the communication system illustratedin FIGS. 1-3) is ramped to a first defined state from the current stateof the line. The current state in this respect is determined by theactivities performed on the line before the procedure of FIG. 5 is run.

The first constant phase achieved is illustrated in a section 18 in FIG.6. For the voltages and currents in FIG. 6, it is assumed that a DSLsignature and a telephone in an on-hook state are present, in otherwords that the tip line A and ring line B are terminated by termination5 and signature 7 illustrated in FIG. 1 connected in parallel.

The voltage Vt on the tip line and the voltage Vr on the ring line areillustrated in the first or topmost graph of FIG. 6. The voltagedifference Vt−Vr and the voltage VCd across the capacitance Cd of DSLsignature 7 of FIG. 1 are illustrated in the second or middle graph ofFIG. 6. Finally, a current ICr flowing across the capacitance Cr oftermination 5 of FIG. 1 and a current ICd flowing across the capacitanceCd are illustrated at the bottom of FIG. 6, in a third graph. In thisrespect, it is assumed that in the illustrated embodiment signature 7 isa standard DSL signature with the Zener diodes Dd having breakthroughvoltages of 6.8 V each.

As illustrated, to assume the first constant phase at 29 in FIG. 5, thevoltage difference Vt−Vr is ramped to the value of 20 V. Furthermore, inthe illustrated embodiment, Vt and Vr are set such that Vt=−Vr.Therefore, during the first constant phase in section 18 of FIG. 6,Vt=10 V and Vr=−10 V.

The voltage VCd in this first constant phase depends on the “history”(i.e., on previous operations performed on the line card) and thereforemay assume other values than the one depicted in FIG. 6.

In the graphs in FIG. 6 and the corresponding analysis, it is assumedthat the resistor Rd of signature 7 of FIG. 1 is 0 (instead of 33 kΩ ina standard DSL signature), and that the forward voltage drop of theZener diodes Dd (i.e., the voltage which drops across the diode biasedin forward bias) is 0 V instead of typically 0.4 to 0.6 V. However,these simplifications do not change the general behavior of the circuitwhich is used for determining whether a DSL signature is present.

During the first constant phase, as soon as a steady state is achievedICr and ICd assume a value of 0 since with a constant voltage basicallyno current flows across the capacitors.

After 29 in FIG. 5, at 30 a first voltage ramp is performed which isdepicted in section 19 of FIG. 6. In this voltage ramp, the voltage isramped from the initial voltage Vt−Vr=V1 to a second voltage V2. Ingeneral, for the measurements to be performed in the illustratedembodiment, V1−V2 should be greater than two times the Zener voltage ofthe Zener diodes Dd, in the present case V1−V2>2·6.8 V=13.6 V.

In the illustrated embodiment V2 is chosen to be −V1 (i.e., −20 V) and aslope of the ramp may of example be chosen as 200 V/s such that the rampfrom V1 to V2 is completed in 0.2 s. During this first ramp depicted insection 19 of FIG. 6, correspondingly Vt is ramped to −10 V and Vr isramped to +10 V.

As further illustrated, VCd basically follows the change of Vt−Vr.

Through the change of the applied voltage, a current flows over thecapacitances Cr and Cd as depicted in the lower part of FIG. 6.

In particular, since the voltages applied are large enough such that thecorresponding Zener diodes are conducting, both a current ICr and acurrent ICd flows. Since the slope of Vt−Vr is constant and the currentflowing across a capacitance is approximately determined by

$\begin{matrix}{I = {C \times \frac{\mathbb{d}V}{\mathbb{d}T}}} & (1)\end{matrix}$wherein I is the current flowing, C is the respective capacitance (Cr orCd in the present case) and dV/dT is the derivative of the appliedvoltage with respect to time (i.e., the slope of the voltage ramp). Asillustrated in FIG. 6, constant currents ICr and ICd are reached after acertain onset time which depends on the capacitance and also onresistances connected thereto.

After a time D longer than the onset time, a current I1 is measuredwhich is the transversal current flowing via tip line A and ring line Bas explained above. For a more precise measurement, an integration overa certain integration time INT is performed.

The onset behavior of the currents ICr and ICd as well as their decayingbehavior is determined byI(t)=Is·(1−exp(−t/T))  (2)for the onset andI(t)=Isexp(−t/T)  (3)for the decaying, wherein I(t) is the respective current (ICr or ICd)depending on time, Is is the final or steady state value as assumed atthe end of section 19 or section 21 of FIG. 6 for the respectivecurrents, t is the time and T is the time constant of the system whichmay be calculated according toT=(Cr+Cd)·Rr  (4)which simplifies toT=Cr·Rr  (5)assuming that Cr>>Cd which is the case for standard DSL signatures andtypical on-hook terminations of telephones.

After 30 in FIG. 5, at 31 the applied voltage is kept constant during asecond constant phase which is illustrated in section 20 of FIG. 6. Inother words, the voltage difference between tip line and ring line Vt−Vris held at V2. In this case, the voltage VCd assumes a constant value ofV2 less the breakthrough voltage of one Zener diode, in the examplegiven where V2 is −20 V and the Zener voltage is 6.8 V VCr would be−13.2 V.

The exact behavior of VCd in section 19 depends on its starting value,which as explained above, depends on the history.

During the second constant phase of section 20, the currents ICr and ICddrop to 0 with their respective time constants since the voltage overthe respective capacitors is not varied and therefore, according toequation (1), no current flows.

After 31, at 32 a second voltage ramp is performed from V2 to theinitial state V1 which is depicted in section 21 of FIG. 6. The sameslope, but with a different sign, as in step 30 is used in theillustrated embodiment. In other words, the ramp performed at 32 is theopposite of the ramp performed at step 30.

During this ramp, VCd at first stays constant at −13.2 V until Vt−Vr haschanged by more than twice the Zener voltage of 6.8 V (Vt−Vr=−6.4 V inthe present case). In this respect, it has to be taken into account thatthe current flowing via the capacitors is determined by the slope of thevoltage ramp and in particular the direction of the current isdetermined by the sign of the ramp, whereas the voltage drop over theZener diodes and its direction is primarily dependent on the voltagemomentarily applied and the sign wherein which, for example at thebeginning of the ramp in section 21, is negative while the slope of theramp is positive, such that the change of Vt−Vr has to be twice theZener voltage before VCd changes.

In this respect, a similar effect would be observed during the firstramp in section 19 if VCd had an appropriate starting voltage.

The varying voltage again causes a current to flow over the capacitorsCr and Cd, wherein the onset of current ICr coincides with the beginningof the ramp, whereas the onset of ICd corresponding with the beginningof the change of the voltage VCd.

At and near the end of section 21, stationary currents are flowing bothacross Cr and Cd. After a delay D which may be the same as delay D insection 19 for reasons explained later, a current I2 is measured duringan integration time INT.

After 32, at 33 the applied voltage V1 is held constant during a thirdconstant phase at 33 which is depicted in section 22 of FIG. 6. Thebehavior in section 2 is similar to the behavior illustrated in section20 (i.e., VCd assumes a value of V1 less the Zener voltage) in theexample given a value of 13.2 V, and ICr and ICd drop to 0 as determinedby the time constant of the system.

After the third constant phase at 33, a third ramp is performed at 34which is illustrated in section 23 of FIG. 6.

In contrast to the first ramp and the second ramp at 30 and 32, an endvoltage V3 which is smaller than the voltage used in the first ramp(i.e., V2) is used. Still, V1−V3>2·6.8 V (the Zener voltage) should bemaintained. In the illustrated embodiment, V3=0 V with a slope of 50 V/sis used.

The behaviour of VCd is similar to the one for the first ramp withreversed sign (i.e., VCd stays constant at 13.2 V until Vt−Vr haschanged by more than twice the Zener voltage and then starts todecrease). Since the slope of the third ramp is considerably less thanthe slope of the first and second ramps, VCd stays constant for a longertime than during the second constant phase.

In particular, since VCd stays constant for a longer time and duringconstant phase no current ICd flows, the current ICr at leastapproximately reaches its steady state before the onset of ICd. After adelay D from the beginning of the third ramp, a current I3 is measuredsuch that the corresponding integration time INT is finished before theonset of ICd. In other words, I3 is determined by ICr, but not by ICd.

After the third ramp at 34 in FIG. 5, at 35 the voltage is held constantin a fourth constant phase 35 depicted in section 24 of FIG. 6. Duringthis constant phase, the current ICr drops to 0. Furthermore, thevoltage VCd assumes a constant voltage of 6.8 V (i.e., the Zenervoltage). Correspondingly, the voltage across the Zener diode Dd whichis reverse biased is −6.8 V such that the overall voltage over thesignature 7 is V3=0 V. Since the voltage across the Zener diode is −6.8V (or very slightly below), the Zener diode is non-conducting and thecharge on capacitance Cr responsible for the voltage of 6.8 V may notdischarge.

After the fourth constant phase at 35, at 36 a fourth ramp is performed.This ramp starts from voltage V3 and leads to a larger voltage V4 suchthat V4−V3<2·6.8 V (the Zener voltage of diodes Dd). In the illustratedembodiment, V4=10 V. The slope of the fourth ramp in the illustratedembodiment has the same magnitude, but the opposite sign from the slopeof the third ramp (i.e., 50 V/s).

In this case, because of the effect of the Zener diodes Dd, the chargeon the capacitance Cd and therefore the voltage VCd remains constant andcorrespondingly no current ICd flows. On the other hand, no such effectis present for the termination 5 of FIG. 1, such that a current ICrflows over the capacitance Cr, which is measured as current I4 after adelay D over an integration time INT.

With this measurement, the measurements required for the embodiment ofFIG. 5 are terminated. Section 26 illustrates that the voltage is keptconstant after this, however, further measurements for other purposeslike the determination of leakage capacitances and resistances may alsofollow.

After 36, at 37 the results are calculated. Before explaining thiscalculation in more detail, it is to be noted that a check like at 27and 28 may be performed also during the execution, as indicated at 38and arrows 44, for example during the constant phases. In this case, themethod is terminated at 39 to be resumed later in order not to disturb auser of the communication line when for example making a telephone call.However, this check may also be omitted in embodiments.

In the following, the calculations performed at 37 are discussed. In thefollowing, S1 designates the slopes of the first ramp and the secondramp (200 V/s in the example given), and S2 designates the slope of thethird ramp and the fourth ramp (50 V/s in the example given).

In this case, the measured currents I1 and I2 are basically given byequation (1). However, in practice offset currents may flow whichinfluence the measurement, such that I1 and I2 are determined asI1=−Ctot·S1+Ioff  (6)I2=Ctot·S1+Ioff.  (7)wherein Ioff designates an offset current and Ctot includes both Cr andCd according toCtot=Cr+Cd  (8)from equations (6) and (7), Ctot may be calculated asCtot=(I2−I1)/(2·S1)  (9)As can be seen, through performing two measurements at two ramps likethe first ramp and the second ramp, the offset current Ioff may becancelled out.

On the other hand, as already explained, the voltages and slopes in thethird and fourth ramp are chosen such that the voltage across Cd doesnot change over a considerable part of the ramp or the whole ramp in thecase of the fourth ramp such that no DC current flows across Cd and themeasured current I3, I4 is only determined by Cr. In particular, in thethird ramp of section 23, the voltage VCd will start from the initialvalue of 30.2 V just after Vt−Vr comes below 20−2·6.8V=6.4V. With a rampslope S2 of 50 V/s, this means that the current measurement formeasuring current I3 has to be executed within the first (2·6.8/50)s=0.272 s. For the measurement of I3 and I4, similar to equations (6)and (7) the following equations determine I3 and I4:I3=−Cr·S2+Ioff  (10)I4=Cr·S2+Ioff  (11)and, similar to equation (8), Cr is then calculated according toCr=(I4−I3)/(2·S2)  (12)finally, the capacitance Cd of the signature 7 can be calculated fromequations (8), (9), and (12) according toCd=Ctot−Cr=(I2−I1)/(2·S1)−(I4−I3)/(2·S2)  (13)

Therefore, if Cd calculated in this way is approximately 475 nF (thestandard capacitance in a DSL signature like the signature 7 of FIG. 1)it can be concluded that a DSL device is present. On the other hand, ifno DSL device is present and therefore also no signature is present,during the first ramp and the second ramp the measured currents I1 andI2 are only determined by the capacitance Cr meaning that in this caseCtot=Cr in equation (8), such that in this case the result for Cdaccording to equation (13) is 0 within the measurement accuracy.

Regarding the measurement accuracy, using Cr in the equationsillustrated constitutes a slight approximation since, as can be takenfrom FIG. 1, a leakage capacitance Ctr is connected in parallel to Cr.However, as long as no fault occurs this leak capacitance is usuallymuch smaller than Cr and therefore negligible. Furthermore, Ctr would beadded to Cr in all the equations and therefore would be cancelled outwhen calculating Cd according to equation (13).

With the values for V1 to V4 and S1 and S2 given above and a ringer load(termination 5) of one U.S. ring equivalent number meaning a capacitanceCr of 8 μF in series with a resistor Rr of 6980Ω, the currents I1 to I4measured will be approximately±200 V/s·(8 μF+470 nF)=±1.7 mA  (14),and the measured currents I3 and I4 will be approximately±50 V/s·8 μF=+0.42 mA  (15)with a standard DSL signature having a capacitance of 470 nF as alreadydescribed. These currents may be easily measured with the embodiment ofFIG. 4.

With the values given as an example for the embodiment of FIGS. 5 and 6,the first ramp and the second ramp each last 200 ms, the third ramplasts 400 ms and the fourth ramp again lasts 200 ms. As alreadyexplained, the onset times of the currents are determined by thecapacitances and also by the resistances. Therefore, the delays D areused for before measurement. For the values given above, a delay time ofapproximately 30 to 40 ms may be used.

In one embodiment, the delays D in sections 19, 21, 23, and 25 are allequal. In this case, by estimating the time constant T in equation (2),the calculation of Cr (see equation (12)) may be corrected according toCrC=Cr/(1−exp(−D/T))  (16)wherein CrC is the corrected value for Cr. This, as a matter of course,also makes the calculation Cd according to equation (13) more exact, andin principle a similar correction may be used for equation (9).

The detection whether an xDSL signature is effectively present may forexample be performed by comparing Cd as calculated by equation (13) witha threshold value, for example a value of 200 nF. If Cd is greater thanthis threshold value, it is decided that a DSL signature and therefore aDSL device is present, otherwise is it decided that no such signatureand device are present. As a matter of course, different thresholdvalues may also be used, and in case a plurality of different signatureswith different capacitances are used in a communication system, Cd mayalso be compared with more than one threshold value to determinedifferent types of signatures.

If, as explained in the embodiment of FIG. 5, steps 27, 28, 38, and 39are performed, it is made sure that the line is idle and therefore in anon-hook state. Consequently, termination 5 is present.

On the other hand, as already indicated these steps may be omitted. Inthis case, if the line is not idle and a corresponding telephone is inan off-hook state, instead of termination 5 of FIG. 1 termination 6 ofFIG. 1 is present. In this case, Cd is the only capacitance (apart fromthe parasitic capacitance Ctr). On the other hand, via termination 6 anadditional DC current may flow depending on the voltage Vt−Vr (but noton the slope of the ramps) which may disturb the measurements. However,by extending the method slightly such an off-hook termination may bedetected and taken into account.

To achieve this, during the third constant phase at 31 and after thefourth ramp at 36, corresponding currents I5 and I6 are measured, I5being measured in the third constant phase (section 22 of FIG. 6) and I6being measured after the fourth ramp (section 26 of FIG. 6) in anembodiment. The measurements in one embodiment are performed near theend of the respective constant phases where any current flowing over apossibly present DSL signature 7 have decayed.

Therefore, the measured currents I are in this case determined by thecurrent flowing via the off-hook termination 6 of FIG. 1 according toI=(V−Vz)/Ro1  (17)wherein I is the measured current (I5 or I6), V is the correspondingvoltage (V1 or V4) and Vz is the Zener voltage of Zener diodes Do1 inFIG. 6. Therefore, Ro1 may be calculated according toRo1=(V1−V4)/(I5−I6)  (18)and the Zener voltage Vz may be calculated according toVz=V1−I5·Ro1=(I5·V4−V1·I6)/(I5−I6)  (19)This calculation according to equations (17) and (18) is depicted FIG.7.

If Vz calculated in this way is close to 0 V, for example <0.2 V, thismeans that the termination of the tip ring wires is a pure resistance.In this case, it is likely that the line is terminated with the shortcutmeaning a “resistive fault” indication. In this case, the line may beshut down in order to repair the line.

In case of a high value for Ro1 and a not neglectable value for Crcalculated during the measurements according to equation (12) (e.g.,Cr>100 nF), the line is terminated with the telephone in the on-hookstate with a high probability within the measurement accuracy (i.e., notterminated by termination 6) but termination 5. On the other hand, ifthe value of Ro1 is not high-omic, for example in the MΩ range, thetelephone is an off-hook state (termination 6 of FIG. 1).

Therefore, with the described extension of the measurements its alsopossible to determine the state of the telephone terminating the line.

The embodiments of FIGS. 5 and 6 may also be modified to additionally oralternatively detect signatures or terminations like the onesillustrated in FIGS. 3A and 3B. In general, corresponding embodimentsuse the fact that the current flowing via these terminations depends onthe polarity of an applied DC voltage. In a corresponding embodimentbased on the embodiments of FIGS. 5 and 6, a current I7 is measuredduring the second constant phase of step 31 (section 20 of FIG. 6), acurrent I8 is measured during the third constant phase at 33 (section 22of FIG. 6), and current I9 is measured during the fourth constant phaseat 35 (section 24 in FIG. 6). As explained before with reference to FIG.7, in one embodiment, the currents are measured at or near the end ofthe respective constant phases to minimize the effect of the decayingcurrents flowing over the capacitances Cr and Cd. Furthermore, in theembodiment discussed since during the fourth constant phase of section24 a voltage V3=0 V is applied, the measured current I9 basicallycorresponds to an offset current.

Based on these measurements, the following resistances Rp and Rn arecalculated:Rp=(V1−V3)/(I8−I9)  (20)Rn=(V2−V3)/(I7−I9)  (21)

The resistance values Rp and Rn thus calculated are then compared with alow threshold value, for example 20 kΩ, and a high threshold value, forexample 1 MΩ. If Rp exceeds the higher threshold value and is thereforeto be considered high-omic and Rn is below the lower threshold value andtherefore low-omic, a termination or signature is present wherein adiode is connected like in FIG. 3B, whereas if Rn exceeds the higherthreshold value and Rp is below the lower threshold value, a diode ispresent like in FIG. 3A. On the other hand, if none of these two casesapply, for example if Rp and Rn are equal or almost equal within themeasurement accuracy, no termination or signature like in FIGS. 3A and3B is present.

The value of the lower threshold value in embodiments is chosen suchthat it exceeds Ro2 and Ro3 in FIG. 2 since, in case the voltage isapplied such that the respective diode Do2 or Do3 is conducting, apartfrom the forward voltage of the diode mainly the resistance of Ro2 orRo3 is measured.

As already explained, the methods described above may be executed withthe embodiment of FIG. 4. In this case, in particular, requiredcalculations and comparisons may be performed by digital signalprocessor 3, or the results may be written for example to a register andread out by a host (not illustrated) via interface 4. In this case, theprocessing may be performed by the host.

FIG. 8 illustrates the voltage Vt applied to the tip line and thevoltage Vr applied to the ring line of a communication line during anexecution of an embodiment like the embodiments of FIGS. 5 and 6 asmeasured by an oscilloscope. In particular, sections 18-26 in FIG. 8correspond to sections 18-26 in FIG. 6. Furthermore, the initializationto the first constant phase at 29 of FIG. 5 is illustrated in moredetail in sections 40 and 41. In particular, when the method is started,some arbitrary voltages are applied to tip line and ring line, in theexample illustrated Vt≈18 V and Vr≈−18 V. Therefore, in section 41 rampsare performed to bring the system to the first constant phase withdefined voltage values. Furthermore, in the embodiment illustrated inFIG. 8 the initial values of section 40 are stored, and in section 42 aramp is performed to bring Vt and Vr to the initial values such thatsection 43 corresponds to the situation of section 40. In thisembodiment, no voltage steps but only ramps are present also from theinitial arbitrary voltage to the first constant phase. In such anembodiment, no ringing noise or clicking noise or the like is induced inolder mechanical telephones, whereas in embodiments with voltage stepssuch noise may be induced.

The present invention is not limited to the embodiments as describedabove, and numerous modifications are possible. For example, in theembodiments of FIGS. 5 and 6, separate voltage ramps are performed formeasuring I1, I2, I3, and I4 which leads to a high precision of themeasurement. On the other hand, it can be seen for example in the secondvoltage ramp of section 21 or the third voltage ramp of section 23 thatduring these ramps in a portion only a current via capacitance Cr flows,whereas in a different portion a current flows both via a capacitance Crand capacitance Cd. Therefore, in a different embodiment twomeasurements may be performed within a single voltage ramp. For example,in a ramp like the third voltage ramp of section 23, a furthermeasurement may be performed at or near the end of the voltage rampwhere both ICr and ICd are non-zero. Such a measurement may for examplereplace the measurement in the first voltage ramp or the second voltageramp. Therefore, it would be possible to shorten the measurementprocedure. In particular, with a further voltage ramp being the reverseof the third voltage ramp, two further measurements may be performed inanother embodiment and then the calculation according to equation (13)is performed. If precision is even less important, in yet anotherembodiment only a single ramp may be used. In this case, the currentoffset Ioff of equations (6), (7), (10), and (11) is not compensated,but the measurement may be performed even faster. As a matter of course,in these embodiments ramps where no measurements are performed may beomitted.

Furthermore, the specific values for voltages and currents given for theembodiments of FIGS. 5 and 6 may be varied. Also, the detection ofvarious signatures or terminations has been discussed with reference toDSL signatures or the signatures or terminations of FIGS. 3A and 3B.However, if it is known beforehand that only a DSL signature 7 of FIG. 1or no signature is present, in a corresponding embodiment only the stepsfor detecting DSL signature 7 have to be performed. The same holds truefor the other terminations and signatures discussed.

Furthermore, the measurement and calculation steps presented do not haveto be performed in the order illustrated. For example, in theembodiments of FIG. 6, the third and fourth voltage ramps for measuringI3 and I4 may be performed first, and after this the first and secondvoltage ramps for measuring I1 and I2. Also, the calculations presentedin the various calculations may be performed, as illustrated in theembodiment of FIG. 5, after all the measurements, or may be performed assoon as the data for making a specific calculation has been measured.

The above modifications are only examples for the numerous modificationswhich may be performed, and further modifications apparent to personsskilled in the art are also considered to be within the scope of theembodiments of the present invention.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method of testing a communication line, themethod comprising: applying a voltage as a function of time on thecommunication line; measuring at least a first current and a secondcurrent flowing via the communication line, wherein the second currentis measured at a different point in time than the first current; anddeciding whether a given terminal element is connected to thecommunication line based on the first current and the second current. 2.The method according to claim 1, wherein the terminal element comprisesat least one of a termination and a signature.
 3. The method accordingto claim 1, wherein the terminal element comprises a non-linear element.4. The method according to claim 3, wherein the non-linear elementcomprises a diode; wherein the measuring of the first current isperformed at a point in time where the diode is forward biased by thevoltage; and wherein measuring the second current is performed at apoint in time where the diode is reversed biased by the voltage.
 5. Themethod according to claim 3, wherein the non-linear element comprises afirst Zener diode and a second Zener diode connected in series withopposite polarities; wherein the first current is measured at a point intime wherein the voltage is at a level that causes the Zener diodes toconduct; and wherein measuring the second voltage is performed at apoint in time wherein the voltage is at a level which causes at leastone of the Zener diodes to be non-conducting.
 6. The method according toclaim 1, wherein the function comprises at least one of a voltage rampand at least one voltage plateau.
 7. A method of testing a communicationline, the method comprising: applying a first voltage ramp to thecommunication line from a first voltage to a second voltage; measuring afirst current during the first ramp; applying a second voltage ramp fromthe second voltage to a third voltage on the communication line;measuring a second current during the second ramp; applying a thirdvoltage ramp from the third voltage to a fourth voltage on thecommunication line; measuring a third current during the third voltageramp; applying a fourth voltage ramp from the fourth voltage to a fifthvoltage on the communication line; measuring a fourth current duringthat fourth ramp; and deciding whether a given terminal element ispresent based on the first current, the second current, the thirdcurrent, and the fourth current.
 8. The method according to claim 7,wherein the given terminal element comprises a first breakthrough diodeand a second breakthrough diode connected in series, wherein the firstbreakthrough diode has a reversed polarity compared to the firstbreakthrough diode.
 9. The method according to claim 8, wherein thefirst current and the second current are measured in a state wherein thefirst breakthrough diode and the second breakthrough diode areconducting; and wherein the third current and the fourth current aremeasured in a state wherein at least one of the breakthrough diodes isnon-conducting.
 10. The method according to claim 8, wherein at leastone of the voltage difference between the first voltage and the secondvoltage, the second voltage and the third voltage, the third voltage andthe fourth voltage, and the fourth voltage and the fifth voltage exceedthe sum of the breakthrough currents of the first breakthrough diode andthe second breakthrough diode; and wherein at least another one of thevoltage difference between the first voltage and the second voltage, thesecond voltage and the third voltage, the third voltage and the fourthvoltage, and the fourth voltage and the fifth voltage are smaller thanthe sum of the breakthrough currents of the first breakthrough diode andthe second breakthrough diode.
 11. The method according to claim 8,wherein the given terminal element comprises a capacitance, wherein afurther terminal element connected to the communication line comprises afurther capacitance, and wherein deciding whether the specified terminalelement is present comprises: calculating a value of the capacitancebased on the first through fourth currents and on slopes of the firstthrough fourth ramps.
 12. The method according to claim 8, wherein thefirst breakthrough diode and the second breakthrough diode each comprisea Zener diode.
 13. The method according to claim 7, wherein the firstthrough fourth currents are measured after a predetermined delay timefrom the beginning of the respective first through fourth ramp.
 14. Themethod according to claim 7, comprising: applying constant voltages tothe communication line between the first through fourth voltage ramps.15. The method according to claim 14, comprising: measuring at least twovoltage/current couples at at least two different constant voltages ofthe constant voltages; and calculating a value of a resistance connectedin series with a pair of breakthrough diodes and connected to thecommunication line based on the first couple and the second couple. 16.An apparatus for testing a communication line, the apparatus comprising:means for applying a voltage as a function of time on the communicationline; means for measuring at least a first current and a second currentflowing via the communication line, wherein the second current ismeasured at a different point in time than the first current; and meansfor determining whether a given terminal element is connected to thecommunication line based on the first current and the second current.17. The apparatus according to claim 16, wherein the terminal elementcomprises a diode; wherein the measuring of the first current isperformed at a point in time where the diode is forward biased by thevoltage; and wherein measuring the second current is performed at apoint in time where the diode is reversed biased by the voltage.
 18. Theapparatus according to claim 16, wherein the terminal element comprisesa first Zener diode and a second Zener diode connected in series withopposite polarities; wherein the first current is measured at a point intime wherein the voltage is at a value that causes the Zener diodes toconduct; and wherein measuring the second voltage is performed at apoint in time wherein the voltage is at a level that causes at least oneof the Zener diodes to be non-conducting.
 19. An apparatus configured totest a communication line, the apparatus comprising: a voltage sourceconfigured to apply a first voltage ramp to the communication line froma first voltage to a second voltage, apply a second voltage ramp fromthe second voltage to a third voltage on the communication line, apply athird voltage ramp from the third voltage to a fourth voltage on thecommunication line, and apply a fourth voltage ramp from the fourthvoltage to a fifth voltage on the communication line; a current meterconfigured to measure a first current during the first voltage ramp,measure a second current during the second voltage ramp, measure a thirdcurrent during the third voltage ramp, and measure a fourth currentduring the fourth voltage ramp; and a calculation unit configured todetermine whether a given terminal element is present based on the firstcurrent, the second current, the third current, and the fourth current.20. The apparatus according to claim 19, wherein the given terminalelement comprises a first breakthrough diode and a second breakthroughdiode in series, wherein the first breakthrough diode has a reversedpolarity compared to the first breakthrough diode.
 21. The apparatusaccording to claim 20, wherein the current meter is configured tomeasure the first current and the second current in a state wherein thefirst breakthrough diode and the second breakthrough diode areconducting, and measure the third current and the fourth current in astate wherein at least one of the breakthrough diodes is non-conducting.22. The apparatus according to claim 20, wherein at least one of thevoltage difference between the first voltage and the second voltage, thesecond voltage and the third voltage, the third voltage and the fourthvoltage, and the fourth voltage and the fifth voltage exceed the sum ofthe breakthrough currents of the first breakthrough diode and the secondbreakthrough diode; and wherein at least another one of the voltagedifference between the first voltage and the second voltage, the secondvoltage and the third voltage, the third voltage and the fourth voltage,and the fourth voltage and the fifth voltage are smaller than the sum ofthe breakthrough currents of the first breakthrough diode and the secondbreakthrough diode.
 23. The apparatus according to claim 20, wherein thegiven terminal element comprises a capacitance; wherein a furtherterminal element connected to the communication line comprises a furthercapacitance; and wherein the calculation unit is configured to calculatea value of the capacitance based on the first through fourth currentsand on slopes of the first through fourth ramps.
 24. A communicationequipment comprising: at least one line card, the at least one line cardcomprising a subscriber line interface circuit configured to couple to acommunication line and a coder/decoder; a programmable control unitcoupled with the line card and comprising storage configured to store aprogram comprising a test procedure, wherein, when the test procedure isrun, the programmable control unit is configured to control thesubscriber line interface and the coder/decoder to perform thefollowing: apply a voltage as a function of time on the communicationline; measure at least a first current and a second current flowing viathe communication line, wherein the second current is measured at adifferent point in time than the first current; and decide whether agiven terminal element is connected to the communication line based onthe first current and the second current.
 25. The communicationequipment according to claim 24, wherein the terminal element comprisesat least one of a termination and a signature.
 26. The communicationequipment according to claim 24, wherein the terminal element comprisesa non-linear element.
 27. The communication equipment according to claim26, wherein the non-linear element comprises a diode; wherein themeasuring of the first current is performed at a point in time where thediode is forward biased by the voltage; and wherein the measuring of thesecond current is performed at a point in time where the diode isreversed biased by the voltage.
 28. The communication equipmentaccording to claim 26, wherein the non-linear element comprises a firstZener diode and a second Zener diode connected in series with oppositepolarities; wherein the first current is measured at a point in timewherein the voltage is at a level that causes the Zener diodes toconduct; and wherein measuring the second voltage is performed at apoint in time wherein the voltage is at a level that causes at least oneof the Zener diodes to be non-conducting.
 29. The communicationequipment according to claim 28, wherein the function comprises at leastone of a voltage ramp and a voltage plateau.
 30. A communicationequipment comprising: at least one line card, the at least one line cardcomprising a subscriber line interface circuit configured to couple to acommunication line and a coder/decoder; a programmable control unitcoupled with the line card and comprising storage configured to store aprogram comprising a test procedure, wherein, when the test procedure isrun, the programmable control unit is configured to control thesubscriber line interface and the coder/decoder to perform thefollowing: apply a first voltage ramp to the communication line from afirst voltage to a second voltage; measure a first current during thatfirst ramp; apply a second voltage ramp from the second voltage to athird voltage on the communication line; measure a second current duringthe second ramp; apply a third voltage ramp from the third voltage to afourth voltage on the communication line; measure a third current duringthe third ramp; apply a fourth voltage ramp from the fourth voltage to afifth voltage on the communication line; measure a fourth current duringthat fourth ramp; and decide whether a given terminal element is presentbased on the first current, the second current, the third current, andthe fourth current.
 31. The communication equipment according to claim30, wherein the specified terminal element comprises a firstbreakthrough diode and a second breakthrough diode in series with thefirst breakthrough diode and with reversed polarity compared to thefirst breakthrough diode; wherein the first current and the secondcurrent are measured in a state wherein the first breakthrough diode andthe second breakthrough diode are conducting; and wherein the thirdcurrent and the fourth current are measured in a state wherein at leastone of the breakthrough diodes is non-conducting.
 32. The communicationequipment according to claim 31, wherein at least one of the voltagedifference between the first voltage and the second voltage, the secondvoltage and the third voltage, the third voltage and the fourth voltage,and the fourth voltage and the fifth voltage exceed the sum of thebreakthrough currents of the first breakthrough diode and the secondbreakthrough diode; and wherein at least one of the voltage differencebetween the first voltage and the second voltage, the second voltage andthe third voltage, the third voltage and the fourth voltage, and thefourth voltage and the fifth voltage are smaller than the sum of thebreakthrough currents of the first breakthrough diode and the secondbreakthrough diode.
 33. The communication equipment according to claim30, wherein the terminal element comprises a capacitance; wherein afurther terminal element connected to the communication line comprises afurther capacitance; and wherein the deciding whether the specifiedterminal element is present comprises calculating a value of thecapacitance based on the first through fourth currents and on slopes ofthe first through fourth ramps.
 34. The communication equipmentaccording to claim 30, wherein the first through fourth currents aremeasured after a predetermined delay time from the beginning of therespective first through fourth ramp.
 35. The communication equipmentaccording to claim 30, wherein, when the test procedure is run, theprogrammable control unit is configured to control the subscriber lineinterface and the coder/decoder to perform the following: apply constantvoltages between the first through fourth voltage ramps; measure atleast two voltage/current couples at at least two different plateaus ofthe plateaus; and calculate a value of a resistance connected in serieswith a pair of breakthrough diodes and connected to the communicationline based on the first couple and the second couple.